The doctrine of having P-51s stay with the bombers all the way to the target as their primary mission, as done during Act IV, actually lasted only a few weeks. Less than a month after the first long-ranged P-51s based in Britain became operational, General James H. "Jimmy" Doolittle took over command of the Eighth Air Force, and one of the first things he did was tell the head of Eighth Air Force Fighter Command to take down a sign at his headquarters that read "THE FIRST DUTY OF THE EIGHTH AIR FORCE FIGHTERS IS TO BRING THE BOMBERS BACK ALIVE" and replace it with one that read " THE FIRST DUTY OF THE EIGHTH AIR FORCE FIGHTERS IS TO DESTROY GERMAN FIGHTERS." As more P-51s were delivered to Eight Fighter Command until all but one of 16 the groups replaced their shorter-ranged P-38s and P-47s with P-51s, proportionately fewer fighter groups were tasked with close-escorting the bombers, with the remaining groups sent out ahead of the bombers to attack the German fighters on the ground or between their airfields and the bombers. This included some of the tactics Lieutenant Colonel Trope employed against orders. This change in doctrine actually decreased US bomber losses by 80%.
This show aired out of order, since it is Part 3 of a story that began in We're Not Coming Back (1965) which aired many weeks later. It is easy to tell due to characters discussing the shuttle mission and referring to actions that occurred in Germany and North Africa.